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Marianne Advanced and Applied > Atomic Force Microscopy (AFM) > Flashcards

Atomic Force Microscopy (AFM) Flashcards

1
Q

How does the AFM work?

A
  1. Laser (probe) aligned on backside of cantilever
  2. Reflected into photodiode.
  3. Probe moves across sample surface.
  4. AFM tip touches surface, records small force between probe and surface.
  5. Image created on computer screen by signals from photodiode.
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2
Q

What are the main properties of the experiment of AFM?

A
  1. Forces between sharp probe, <10 nm
  2. Distance between tip and surface, 0.2 - 10 nm
  3. Resonance frequencies more than 100 kHz
  4. mass of cantilever, m = 1 μg
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3
Q

How are the forces measured in AFM?

A
  1. Probe place in the end of cantilever
  2. Amount of force between probe and sample depend on spring constant (stiffness)
  3. Hooke’s Law: F = -k * x
  4. If spring constant less than surface, cantilever bends, deflection is monitored.
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4
Q

What are the measurements of the forces in open air?

A

nN (10^-9) to μN (10^-6)

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5
Q

What types of forces are measured in AFM?

A
  1. Repulsive Van der Waals forces (when in contact mode)
  2. Attractive Van der Waals forces (non-contact mode)
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6
Q

What are the two modes in AFM?

A
  1. Contact mode AFM (repulsive VdW)
  2. Intermittent mode (Tapping)
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7
Q

What is contact mode AFM?

A
  • When the spring constant of the cantilever is less than surface, causing the cantilever to bend.
  • The force on the tip is repulsive.
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8
Q

What are advantages of contact mode AFM?

A
  1. fast scanning
  2. good for rough samples
  3. used for friction analysis
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9
Q

What are disadvantages of contact mode AFM?

A
  1. force can damage soft samples
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10
Q

What is intermittent mode AFM?

A

When the cantilever is oscillated at resonant frequency.
The probe taps on the sample surface during scanning.

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11
Q

What are advantages of intermittent mode AFM?

A
  1. Allows high resolution of samples
  2. good for biological samples
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12
Q

What are disadvantages of intermittent mode AFM?

A
  1. More challenging to image in liquids
  2. Slower scan speeds needed
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13
Q

What are some properties of contact mode probes (tips)?

A
  1. materials: Si3Ni4
  2. Pyramidal shape (radius 10 - 50 nm)
  3. Cantilever length: 50 -500 μm
  4. Spring constant: k ~ 0.1 - 0.7 N/m
  5. Used both in air and liquid medium
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14
Q

What is one other area the contact mode probe can be used?

A

For intermittent contact in liquid, with resonant frequency <20 kHz

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15
Q

What are the differences between AFM and Microscopes?

A

Microscopes:
1. Only generate 2D images
2. Low magnification
Optical: 1000X magnification
Electron: 100.000X

AFM:
1. Measure in 3D (Z-axis, vertical) - shows true topography
2. High magnification: 1.000.000X
(due to sharp tip probing surface features by raster scanning)

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16
Q

What are the advantages of AFM?

A
  1. No vacuum required
  2. Can image non-conducting surfaces
  3. Can be used with liquid samples
  4. More topographical contrast
  5. 3D images
17
Q

What are the disadvantages of AFM?

A
  1. Image size
  2. Slow scanning speeds
  3. Artefacts
18
Q

How do you use AFM to find particles through the images?

A

If particles are assumed to be spherical, the height measurement corresponds to the diameter or size of the particle.

19
Q

What does the Tg depend on in AFM?

A

Aromatic and polar groups.

20
Q

How does the AFM-FTIR work?

A
  1. Sample placed on IR transparent prism (e.g. ZnSe)
  2. Irradiated with total internal reflection style illumination scheme
  3. IR laser tuned to wavelength corresponding to absorption
  4. Absorbed light converted into heat, resulting in thermal expansion of absorbing region.
  5. Detected with tip, signal proportional to absorption coefficient
  6. Absorption spectra obtained
21
Q

When does the thermal expansion in AFM-FTIR occur?

A

When molecular vibrations of the sample excited by absorbed IR photons return to ground state through transfer of energy in form of heat.

Marianne Advanced and Applied (17 decks)

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